There's probably a practical reason o do with playing other roles but, in show, I kinda like the idea a change in his appearance is forced on him once a month but he still has autonomy and can change it himself once a week. There's a traditional connection between women and werewolves (Luna is a female goddess; women have a four-week menstruation cycle; werewolves change during a full moon) - hence the Ginger Snaps films, Company of Wolves and (I think) Red Riding Hood. In BtVS much is made of women changing their hair but Giles, Xander, Spike and Angel have the the same hairstyle throughout. In fact, Spike connects Angel's use of hair product to "aberrant" masculinity or homosexuality - he "feminizes" him. Oz isn't a "traditional" male and it suggests that, despite this, he's comfortable in his own skin (pun intended).